This invention relates to recovery of liquid and gaseous products from oil shale. The term "oil shale" as used in the industry is in fact a misnomer; it is neither shale, nor does it contain oil. It is a sedimentary formation comprising marlstone deposit with layers containing an organic polymer called "kerogen" which upon heating decomposes to produce hydrocarbon liquid and gaseous products. The formation containing kerogen is called "oil shale" herein, and the hydrocarbon liquid product is called "shale oil".
The presence of large deposits of oil shale in the Rocky Mountain region of the United States has given rise to extensive efforts to develop methods of recovering shale oil from kerogen in the oil shale deposits. A number of methods have been developed for processing the oil shale which involve either first mining the kerogen bearing shale and processing the shale on the surface, or processing the shale in situ. The latter approach is preferable from the standpoint of environmental impact inasmuch as the spent shale remains in place, reducing the chance of contamination and the need to dispose of solid wastes.
The recovery of liquid and gaseous products from a subterranean formation containing oil shale has been described in several issued patents, one of which is U.S. Pat. No. 3,661,423, issued May 9, 1972, to Donald E. Garrett, assigned to the assignee of this application and incorporated herein by this reference. That patent describes the in situ recovery of liquid and gaseous carbonaceous products from subterranean formations containing oil shale by preparing an in situ oil shale retort in the subterranean formation. The retort is formed by excavating a production tunnel or drift in the subterranean formation, mining a void in the formation within the boundaries of the in situ retort site, and explosively expanding formation toward the void. This forms a fragmented permeable mass of formation particles containing oil shale, referred to herein as an in situ oil shale retort. Hot retorting gases are passed through the in situ oil shale retort to convert kerogen contained in the oil shale to liquid and gaseous products.
The liquid and gaseous products are cooled by the cooler formation particles in the retort on the advancing side of the retorting zone. The liquid hydrocarbon products, together with water, are withdrawn from the bottom of the retort through a production drift. A process off gas is withdrawn from the bottom of the retort through the production drift. The off gas can contain nitrogen, hydrogen, carbon monoxide, carbon dioxide, water vapor, methane and other hydrocarbons, and sulfur compounds such as hydrogen sulfide. Hydrogen sulfide and carbon monoxide are extremely toxic gases. For this reason it is desirable to prepare in situ oil shale retorts so that workers in a retort preparation or development region of the formation are isolated from the off gas in a retorting region of the formation.
It is necessary to constantly provide a supply of fresh air to workers in a retort preparation region of the formation. The particular method used for forming a system of in situ retorts can contribute to the effectiveness and cost of ventilating underground workings when preparing a system of in situ retorts.
A technique for preparing a system of in situ oil shale retorts is described in U.S. Pat. No. 3,001,776 to Van Poollen. That patent describes techniques for forming retorts involving sublevel stoping, shrinkage stopes, sublevel caving or block caving.
Other techniques are disclosed in U.S. patent application Ser. No. 603,704, filed Aug. 11, 1975, now U.S. Pat. No. 4,043,595, and Ser. No. 659,899, filed Feb. 20, 1976, now U.S. Pat. No. 4,043,598, both of which are assigned to the assignee of this application. These applications are incorporated herein by this reference.
It is desirable to develop a safe and economical system for preparing in situ oil shale retorts. Such a method should leave a minimal amount of unfragmented formation and form retorts which can be operated without difficulty.
In carrying out retorting operations it is desirable to isolate the in situ oil shale retorts from one another so that operations in one retort do not affect those in adjacent retorts. In preparing a system of in situ retorts, it is desirable to provide an effective and inexpensive method for isolating the retorts from one another.